The present invention relates to movable plug valves for controlling the flow of fluid through a conduit, and more particularly to rotary plug valves used to control the flow rate of fluid in pipe lines.
Rotary plug valves generally include a hollow tubular body which is inserted between two sections of pipe or other conduit. The closure mechanism of the valves generally comprises a valve seat or seat ring in one end of the body that cooperates with a spherical plug to seal or close the valve. The plug is attached to an arm which is connected to an input shaft which is arranged transversely to the pipe center line so that as the shaft is rotated by an external means, the plug swings toward or away from the valve seat to modulate the flow rate. The input shaft centerline is usually offset from the valve seat centerline so that the plug will tend to pull away from the seat during opening to thereby reduce the tendency of the part to rub and wear.
A basic problem in the design of rotary plug valves is that the plug must be aligned precisely with the valve seat when the two parts are brought together in order to produce low fluid leakage. That is, when the input shaft is rotated to move the plug towards the valve seat and into engagement therewith, the spherical seating surface of the plug must be capable of being aligned precisely with the peripheral seating surface of the seat to completely close off the flow with minimal leakage.
One type of prior art arrangement for accomplishing this alignment included designing the arm, plug, or seat to bend or flex to allow the parts to be forced into alignment by the input mechanism. With such prior art arrangements, it was desirable to have the parts flex into alignment with a minimum force input in order to utilize smaller, less expensive actuators. This in turn required that the spring rate of the flexing parts be low. However, low spring rates can in turn lead to vibrations of the parts in the open position as a result of fluid turbulence which thus could compromise the reliability of the assembly. Furthermore, designing parts to be flexible also reduced the capability of the parts to carry the required loads, which thus tended to increase the probability of fatigue failure.
Other prior art arrangements for accomplishing the alignment of the spherical plug with the valve seat included machining parts to very close tolerances and/or adjustment of the position of the parts at assembly. However, such prior art arrangements are also disadvantageous in that they are expensive and complicate remachining of worn parts. Furthermore, they do not take into account or compensate for possible dimensional changes due to temperature and/or wear which otherwise might cause leakage.
Further, while other prior art valve arrangements disclose flexible coupling arrangements for joining or coupling the components together, these prior art arrangements introduce additional problems into the design of the plug valves. For example, U.S. Pat. No. 1,487,815 entitled "Poppet Valve", issued Mar. 25, 1924, discloses a poppet valve having a valve head capable of tilting movement in relation to the valve stem so that when the valve is forced into place upon its valve seat, the head will automatically adjust itself to properly seat or align itself with the valve seat. However, that arrangement introduces an additional fluid leakage path through the coupling mounting (i.e., between the valve head and the valve stem) which thus is unacceptable in systems where it is desired to minimize the fluid leakage when the valve is in its closed position.